Heat exchanger tube configuration for improved flow distribution

ABSTRACT

A microchannel heat exchanger includes for each channel, a serpentine shaped tube for providing a plurality of parallel flow passes for successively conducting fluid flow therethrough, and being fluidly interconnected between an inlet and an outlet manifold. Multiple circuits are obtained by the individual serpentine shaped tubes. Various methods are provided for forming the serpentine shaped tubes.

CROSS REFERENCE TO RELATED APPLICATION

This application is a National Stage filing under 35 U.S.C. §371 of PCTApplication No. PCT/US2009/033141, filed Feb. 5, 2009. This applicationalso claims the benefit of U.S. Provisional Application Ser. No.61/034,503 filed Mar. 7, 2008. The entirety of both applications isincorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to air conditioning systems and, moreparticularly, to parallel flow heat exchangers.

BACKGROUND OF THE INVENTION

Refrigerant maldistribution in refrigerant system evaporators is a wellknown phenomenon. It causes significant evaporator and overall systemperformance degradation over a wide range of operating conditions.Maldistribution is particularly pronounced in parallel flow evaporatorsdue to their specific design with respect to refrigerant routing.Attempts to eliminate/reduce the effects of this phenomenon on theperformance of brazed aluminum heat exchangers have been made withlittle or no success. The primary reasons for such failures havegenerally been complexity/inefficiency or prohibitively high cost of thesolution.

In recent years, parallel flow heat exchangers have received muchattention and interest, not just in the automotive industry but also inthe heating, ventilation, air conditioning and refrigeration (HVAC&R)industry. The primary reasons for the employment of the parallel flowtechnology deals with its superior performance, high degree ofcompactness and enhanced resistance to corrosion. Parallel flow heatexchangers are now utilized in both condenser and evaporatorapplications for multiple products and system designs/configurations.The evaporator applications, although promising greater benefits andrewards, are more challenging and problematic. Refrigerantmaldistribution is one of the primary concerns and obstacles for theimplementation of this technology in evaporator applications.

As known, refrigerant maldistribution in parallel flow heat exchangersoccurs because of unequal pressure drops inside the mini-channels ormicrochannels as well as in the inlet and outlet manifolds. In themanifolds or headers, the difference in length of refrigerant paths,phase separation, gravity and turbulence are the primary factorsresponsible for maldistribution. Inside the heat exchangermini-channels, variation in the heat transfer rate, airflow rate andgravity are the dominant factors. Because it is extremely difficult tocontrol all these factors many of the previous attempts to managerefrigerant distribution, especially in parallel flow evaporators, havefailed.

In the refrigerant systems utilizing parallel flow heat exchangers, theinlet and outlet headers usually have a conventional cylindrical shape.When the two-phase flow enters the header, the vapor phase is usuallyseparated from the liquid phase. Since both phases move independently,refrigerant maldistribution tends to occur.

The problems of unequal flow distribution are particularly evident inmulti-pass mini-channel heat exchangers wherein the inlet and outletheaders are commonly divided into longitudinally spaced sections whichare interconnected by straight tubes. One approach to solving theseproblems is shown and described in U.S. Pat. No. 7,143,605, wherein aninlet manifold includes an internally disposed distribution tube with aplurality of orifices formed therein.

Serpentine, multiple pass heat exchangers are known in the art as shownby U.S. Pat. Nos. 7,069,980; 4,962,811; 5,036,909; 6,705,386 and U.S.2005/0217834 A1. Generally, they do not incorporate the feature ofmultiple circuits. U.S. Pat. No. 5,036,909 does include multiplecircuits but they are constructed to be in a nested, one inside theother, relationship. Such a design presents problems of inflexibility indesign, manufacture and use. The present invention overcomes theseproblems.

DISCLOSURE OF THE INVENTION

Briefly, in accordance with one aspect of the invention, the pluralityof parallel mini-channels are serpentine in shape so as to therebyprovide a plurality of parallel flow passes but which are connected tothe inlet and outlet manifolds only at the respective inlet and outletends. In this way, the inlet manifold can be relatively short and bedirectly connected to fewer inlet ends of the microchannels for uniformflow distribution. Further, each circuit has all of its flow passeslaterally spaced from all of the flow passes of the adjacent circuits.

In accordance with another aspect of the invention, a method ofpromoting uniform refrigerant flow from an inlet manifold to a pluralityof parallel mini-channels, including the steps of providing a flat tubeshaped in a serpentine manner to form a plurality of flow passes forsuccessively conducting fluid flow therethrough and fluidly connectingan end thereof to an inlet manifold and the other end thereof to anoutlet manifold, with each circuit having all of its flow passes spacedlaterally from all of the flow passes of the adjacent circuits.

In the drawings as hereinafter described, preferred and modifiedembodiments are depicted; however, various other modifications andalternate constructions can be made thereto without departing from thespirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a multi-pass microchannel heatexchanger in accordance with the prior art.

FIG. 2 is a perspective view of single three pass parallel mini-channelmember in accordance with the present invention.

FIG. 2A is a perspective view of a single four-pass parallelmini-channel member in accordance with the present invention.

FIG. 3 is a perspective view of a single component thereof.

FIG. 3A is an alternative embodiment thereof.

FIG. 4 is an exploded view of components of another embodiment thereof.

FIG. 4A is an alterative embodiment thereof.

FIG. 5A is a schematic illustration of a heat exchanger in accordancewith the prior art.

FIG. 5B is a schematic illustration of a heat exchanger in accordancewith the present invention.

FIG. 6 is an alternative embodiment thereof.

FIG. 7 is yet another alternative embodiment thereof.

FIGS. 8A, 8B and & 8C are schematic illustrations of various possibleembodiments of the inlet manifold.

DETAILED DESCRIPTION OF THE INVENTION

A multi-pass mini-channel heat exchanger in accordance with the priorart is shown in FIG. 1 and includes a primary manifold 11, a secondarymanifold 12 and a plurality of mini-channel tubes 13 fluidlyinterconnected therebetween. The primary manifold 11 has dividers 14 and16 provided therein to thereby form independent sections 17, 18 and 19that are fluidly isolated from each other. The section 17 functions asan inlet manifold and the section 19 functions as an outlet manifold.Similarly, the secondary manifold 12 has a divider 21 which forms thesections 22 and 23 which are so mutually isolated.

The heat exchanger as shown comprises a four pass, seven circuitconfiguration. That is, there are seven tubes in each of the four passgroupings 24, 26, 27 and 28. The tubes in the pass grouping 24 thusfluidly interconnects the section 17 of the primary manifold 11 to thesection 22 of the secondary manifold 12, with the pass grouping 26 thenfluidly interconnecting the section 22 to the section 18 of the primarymanifold. Similarly, the pass grouping 27 fluidly interconnects thesection 18 in the primary header 11 to the section 23 of the secondarymanifold 12, and the pass grouping 28 fluidly interconnects the section23 of the secondary manifold 12, to section 19 of the primary manifold11. The refrigerant then flows through the assembly as indicated by thearrows.

It should be understood that, with such a configuration, uniformdistribution of refrigerant flow to the individual channels is verydifficult to obtain. The primary reason is that the distribution to theseven tubes has to be made at the entrance of each of the pass groupings24, 26, 27 and 28. During each pass transition, such as in section 22,two-phase mixture exiting pass grouping 24 will be allowed to mix, andwill have the tendency to phase separate, leading to maldistribution topass grouping 26. It should be pointed out that, as in the conventionalconfiguration, the mini-channel tubes are spaced with fins in between.

In FIG. 2 there is an illustration of a single parallel mini-channeltube that is applied to obtain a three pass heat exchanger. It comprisesthree planar portions 29, 31 and 32 and the two arcuate portions 33 and34. The planar portions 29, 31 and 32 are arranged in parallelrelationship, with the planar portions 29 and 31 being fluidlyinterconnected by the arcuate portion 33, and with the respective endsof the planar portions 31 and 32 being fluidly interconnected by thearcuate portion 34. An inlet end 36 is fluidly connected to an inletmanifold, and the outlet end 37 is fluidly connected to an outletmanifold. Thus, the refrigerant passes from the inlet manifold andthrough the entire three passes to the outlet manifold without requiringany redistribution of the refrigerant when entering the next pass.

It should be understood that the flat tube structure as shown representsa single circuit in a three pass configuration, and a multi-circuit heatexchanger can be obtained by simply juxtaposing other identically shapedtubes in parallel relationship with the tube as shown. These featureswill be more fully described hereinafter.

It should be recognized that although the tube is shown as being flat inits configuration, it may be formed in other shapes such as round, oval,or racetrack shaped in cross-section, for example. An advantage to theflat shape as shown is that this is conventional geometry formicrochannel or mini-channel heat exchangers. Further, the flat tubesenable the design of a small inactive heat exchanger area at the top andbottom due to their flat profile.

The tube as shown in FIG. 2 represents a finished three-pass tube whichmay be fabricated by any of various manufacturing processes. One methodthat can be applied is to simply form the three pass tube from a singleunitary member which is bent around to form the 180° turns at thearcuate portions 33 and 34. With such an approach, care must be takennot to crimp the tube so as to restrict the flow of refrigerant throughthe arcuate portions 33 or 34. The distance between the planar portions29, 31 and 32 can be selected to fit the design of the overall heatexchanger.

FIG. 2A shows another tube which is formed in a four-pass configurationwith combination of two long bends and one short bend. Here, it will beseen that the bends are substantially 90° bends rather than curvilinearbends as shown in FIG. 2. Accordingly, the considerations for preventingcrimping are different and probably more critical than with the arcuatesections of the FIG. 2 embodiment. Critical in this regard is the typeof material that is used (e.g. preferably a more ductile material), thebend radius, the wall thickness, and the internal parallel arrangementsinside the tubes, which are all factors that can influence the bendshape and form.

Another approach to fabrication is that shown in FIG. 3 wherein ashorter section of tube is bent around a 180° turn near its one end toform a J-shaped member 38 comprising a planar element 39 and an arcuateelement 41. This provides only a single pass from the inlet manifold 42but can easily be combined with other similar J-shaped members to obtaina multi-pass arrangement. That is, to add a second pass to that shown,one can easily connect an end of the planar element 39 of a secondJ-shaped member to the end of the arcuate element 41 of the member asshown to obtain a second pass. A third pass then can be obtained byconnecting a planar element to connect its one end to the end of thearcuate element 41 of the second J-shaped member, with the other endthereof being fluidly connected to the outlet manifold. Connectionsbetween individual members can be made by brazing or the like.

Another possible fabricating process that may be used that shown in FIG.4 wherein the arcuate sections 45 and 43 may be formed from shorterportions of a tube and then connected to the planar elements to obtain athree pass tube. That is, arcuate section 45 fluidly interconnects theends of planar elements 44 and 46, and arcuate section 43 fluidlyinterconnects the ends of planar elements 46 and 47.

The applicants have recognized that, as the refrigerant is expanded asit successively flows through the various passes, it is desirable toprogressively increase the cross sectional areas of the tubes in thedownstream direction. Ideally, this would be accomplished on acontinuous basis but, as a practical matter, such a design would bedifficult to implement. Accordingly, this may also be accomplished in astep wise manner. Such a step wise approach can easily be implemented inthe methods of fabrication as shown in FIGS. 3 and 4 by graduallyincreasing the cross sectional area of the successive planar elementswithin any particular circuit as shown in FIGS. 3A and 4A.

Considering now the manner in which the tubes may be combined to form amultiple circuit heat exchanger, a prior art, nested, approach is shownin FIG. 5A wherein circuits 48 and 49 are fluidly interconnected betweeninlet header 51 and 52. Each of the circuits 48 and 49 is formed in aserpentine shape so as to provide five passes between the inlet header51 and the outlet header 52. This arrangement allows the headers 51 and52 to be relatively small with the inlet header 51 providing for asingle distribution between the two circuits, and with the distributionin each circuit remaining throughout the flow of refrigerant through theheat exchanger. However, in order for the tubes of the circuit 49 to benested within the tubes of the circuit 48 as shown, their size/shapeneeds to be selected accordingly. Further, if one wants to add a thirdcircuit, it would be necessary to provide a third differently shapedtube that could be nested outside of the circuit 48 or inside thecircuit 49. Such a change, in turn, may require the redesigning of theentire heat exchanger when considering the features of the fin density,fin height, tube details, etc.

Referring to FIG. 5B, the heat exchanger of the present invention isshown to include circuits 53 and 54, with each having five passesbetween the inlet header 56 and outlet header 57. However, rather thanhaving the tubes of the circuit 54 nested within the tubes of thecircuit 53 as in the prior art, the entire five passes of the circuit 54are grouped together with the group being laterally spaced from theentire group of five passes of the circuit 53. This arrangement allowsthe tubes of the circuit 54 to be substantially identical to the tubesof the circuit 53, with only the lengths of the inlet lines 58 and 59and the lengths of outlet lines 61 and 62 being different. That is, thefive passes of the circuit 53 are substantially identical to the fivepasses of the circuit 54. This allows them to be mass produced to reducecost. It also allows them to be stacked vertically, horizontally or inthe airflow directions for optimal performance. Further, additionalcircuits can be easily added by simply placing one or more circuits inspaced relationship to the circuit 54.

In FIG. 6 there is shown an alternative embodiment of a heat exchangerhaving a five pass, four circuit arrangement to again obtain a total oftwenty tubes. Here, the four circuits 63, 64, 66 and 67 are fluidlyconnected between an inlet header 68 and an outlet header 69, with eachof the circuits containing five groups of passes between its inlet andoutlet ends.

Referring now to FIG. 7, there are shown two heat exchanger units 70 and71 in spaced relationship with respect to the direction of airflowtherethrough. Unit 70 has circuits 72 and 73 fluidly interposed betweeninlet header 74 and outlet header 76. Unit 71 has circuits 77 and 78fluidly connected between inlet header 79 and outlet header 81. As willbe seen, the inlet and outlet headers of the respective units 70 and 71are substantially reversed. The purpose is to obtain better efficiencywhen considering the operation of the two units in combination. That is,in the heat exchanger unit 70, the refrigerant entering from the leftside of each of the circuits 72 and 73 will tend to be cooler than therefrigerant near to the downstream ends of those circuits (i.e. towardthe right side). Similarly, with the inlet header 79 on the right sideof the unit 71, the refrigerant flowing in the passes nearer to theright side of circuits 77 and 78 will be cooler than the refrigerant inthose passes on the left side of those circuits. Because of thiscounterflow relationship between the flow in the units 70 and 71, a morebalanced heat transfer and better efficiency will result. Thearrangement of circuits as set forth in the present invention facilitiessuch a design.

The applicants have recognized that if a heat exchanger is arranged insuch a manner that the tubes emanating therefrom are in a parallelhorizontal arrangement, but with the tubes being vertically spaced, thengravity will tend to cause more of the heavier liquid refrigerant toflow to the lower tubes and more of the lighter vapor to the uppertubes, thereby causing maldistribution. Accordingly, one of thearrangements of 8A, 8B or 8C is preferable, wherein the inlet manifoldis shown at 82 and the mini-channels are shown at 83. As will be seen inFIGS. 8A and 8B, the incoming fluid is flowing upwardly or downwardly,respectively, and therefore each of the tubes is effected the same asthe other tubes with respect to the force of gravity. Thus, evendistribution is more likely to occur.

In FIG. 8C, in order to further enhance the uniform distribution ofrefrigerant to the tubes 83, a distributor 84 is installed within theinlet header 82 as shown.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be effected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A heat exchanger of the type having at least one unit having inletand outlet manifolds fluidly interconnected by a plurality of circuitswith each circuit having separate parallel mini-channels for conductingthe flow of refrigerant therebetween; wherein said parallelmini-channels are each formed in a serpentine shape so as to provide aplurality of parallel flow passes for successively conducting fluid flowtherethrough and with each circuit having an inlet end fluidly connectedto the inlet manifold and an outlet end fluidly connected to the outletmanifold and with each circuit having all of its parallel flow passesgrouped together, and with each group being laterally spaced from all ofthe groups of the adjacent circuits.
 2. A heat exchanger as set forth inclaim 1 wherein said parallel mini-channels are each formed of a unitarymember which is bent into the desired serpentine shape.
 3. A heatexchanger as set forth in claim 1 wherein said parallel mini-channelsare formed from a plurality of planar tubes with U-shaped membersinterconnected at the ends of adjacent planar tubes to provide theserpentine shape.
 4. A heat exchanger as set forth in claim 1 whereinsaid parallel mini-channels are formed, in part, by fluidlyinterconnected J-shaped members.
 5. A heat exchanger as set forth inclaim 1 wherein said plurality of parallel flow passes have crosssectional areas which increase or decrease toward the downstream passes.6. A heat exchanger as set forth in claim 5 wherein said increases ordecreases are in a step wise fashion.
 7. A heat exchanger as set forthin claim 1 wherein said inlet manifold includes a distributor disposedtherein to facilitate the uniform distribution of refrigerant to theindividual mini-channels.
 8. A heat exchanger as set forth in claim 1wherein said parallel mini-channels have their respective inlet endsoriented vertically.
 9. A heat exchanger as set forth in claim 1including a pair of units arranged in spaced relationship in thedirection of airflow therethrough and with the respective directions ofrefrigerant flow being in counterflow relationship.
 10. A method ofpromoting uniform refrigerant flow from an inlet manifold of a heatexchanger to a plurality of parallel multi-channel, mini-channelsfluidly connected thereto, comprising the steps of: providing aplurality of tubes shaped in a serpentine manner and arranged to form aplurality of circuits with each circuit having a plurality of parallelflow passes for successively conducting fluid flow therethrough and witheach circuit having all of its parallel flow passes grouped together,and with each group being laterally spaced from all of the groups of theadjacent circuits; and fluidly connected each circuit at one end thereofto an inlet manifold and at the other end thereof to an outlet manifold.11. A method as set forth in claim 10 wherein said at least one flattube is formed of a unitary member which is bent into the desiredserpentine shape.
 12. A method as set forth in claim 10 wherein said atleast one flat tube is formed from a plurality of planar tubes withU-shaped members being interconnected at the ends of adjacent planartubes to provide the serpentine shape.
 13. A method as set forth inclaim 10 wherein said parallel mini-channels are formed, in part, byfluidly interconnecting J-shaped members.
 14. A method as set forth inclaim 10 wherein said plurality of parallel flow passes have crosssectional areas which increase or decrease toward the downstream passes.15. A method as set forth in claim 14 wherein said increases ordecreases are in a step wise fashion.
 16. A method as set forth in claim10 wherein said inlet manifold includes a distributor disposed thereinto facilitate the uniform distribution of refrigerant to the individualmini-channels.
 17. A method as set forth in claim 10 and including thestep of orienting the inlet ends of said plurality of flat tubesvertically with respect to one another.
 18. A method as set forth inclaim 10 and including the steps of providing another such heatexchanger in spaced relationship in the direction of air flow to saidone heat exchanger and causing the respective directions of refrigerantflow through the heat exchangers to be in counterflow relationship.